1. Field of the Invention
This invention relates to fire extinguishing apparatus and, more particularly, to a water curtain method and apparatus for stopping the spread of a fire and for extinguishing the fire in an underground coal mine.
2. Description of the Prior Art
Coal is mined because it will burn and, therefore, is a major source of economical energy. In 1994 a little over one billion tons of coal were mined in the United States. Coal was the energy source for over 55% of the electrical energy generated in this country during 1994.
Unfortunately fires can and do occur in underground coal mines. These fires are a threat to the lives of the miners. Fires can cause great financial loss to the mining companies. According to the Mining Safety and Health Administration, there were 271 reportable fires in underground coal mines in the United States during the ten year period from Jan. 1, 1978 through Dec. 31, 1987. Fortunately most of these fires were handled promptly so the losses were small, but some were not.
The December 1984 fire at the Wilburg Mine near Huntington, Utah was especially tragic because it killed 27 miners. The cost of this fire has been variously estimated to have been in excess of 50 million dollars. The 1987 fire at Cumberland Mine of United States Steel Co. closed the mine for many months. It was reported that the cost of fighting the fire and restoring the mine to operation was about 13.5 million dollars. The fire during March of 1988 at the Beth Energy No. 58 Mine near Marianna, Pa. resulted in the total loss of the mine. Bethlehem Steel Co. wrote off 45 million dollars.
This listing of three very serious fires is far from complete, but they do fit the experience of the past. Most fires in coal mines follow a similar pattern. Usually, the start of a fire is small and can be controlled with a fire extinguisher, if it is used soon enough. Today's coal mines are spread over very large areas and much of the area is unattended so a fire may not be discovered promptly. If the fire has enough time to spread to the coal, then the fire has become a mine fire and the fuel for a mine fire is virtually without limit.
Burning coal can be effectively and quickly controlled with water applied as a fire hose stream. If the fire is not discovered quickly or if the fire fighting was not started in good time, this may not work. Fire hose streams have limited range in the restricted height of a mine. The fire almost always takes the course that the ventilation follows. It can only be approached from the upwind side. The downwind side of a fire that has spread to the coal is full of smoke and very hot.
The heat of a mine fire weakens the mine roof and may cause caving. Falls of the roof can prevent the fire hose streams from reaching the fire extending beyond. Also, the firefighters must support the weakened roof as they advance into the area where they have extinguished the fire with their hose streams. Supporting the weakened roof to make it safe is slow, difficult work. Usually the firefighters cannot follow a fire as fast as it moves with the ventilation air. A fire cannot be controlled with fire hose streams when the water does not reach all of the fire.
At this point it may be useful to explain some of the basic fundamentals of coal mining so that persons who have never been in a mine will have a better understanding of the problems of controlling a fire. All underground mines (not just coal mines) must have moving fresh air so that the miners can work. The mining of coal almost always releases methane gas from the coal seam. Breaking the coal from the seam, which really is the mining operation, may release combustible methane gas. Air movement or ventilation is required to dilute the methane to a safe noncombustible concentration and carry the methane-air mix to the surface.
The coal mine is ventilated with "intake air" through a network of "entries" or tunnels in the coal seam. When the intake air reaches the "face" where the actual mining takes place and dilutes and carries away the methane, the air is then called "return air". Return air is directed into "returns" or "return entries" which conduct the return air to the surface.
The return air is not allowed to contact equipment that might ignite a methane-air mix. The only equipment allowed in the face area and in a return must be "permissible" which means that it has been tested and found to be flameproof. Equipment that will be used only in intake entries does not have to be permissible, but much of the electrical equipment will be dusttight.
It should be apparent that in mining or developing entries, there must be at least two entries, an intake and a return, which must have ventilation. Also, in the production area or "section" where the actual mining is taking place there must be connections for the air to flow between the intakes and the returns. These connecting entries are called "crosscuts".
Crosscuts are usually spaced about every 100 feet. When the entries have advanced about 100 feet beyond a crosscut, the next crosscut will be started. When this crosscut is finished, the one behind will be closed to ventilation, eventually with a concrete block or metal wall called a "stopping". In actual practice, there are almost always three or more entries in a section, usually about 100 feet apart, driven with crosscuts between entries.
Modern coal mines can be classified by the mining system used to extract the coal. Virtually all coal mines use powerful "continuous miners" to drive openings in the coal seam.
These machines break the solid coal into pieces so that it can be hauled and/or conveyed to the surface. The openings made are the entries and crosscuts referred to above. Two of the most used mining systems are the longwall mining system and the block or room and pillar system.
Development, or the driving of entries and crosscuts, for the longwall system usually requires driving 4, 3, or sometimes only 2 entries with the necessary crosscuts between these entries. Since the entries have different functions and ventilation, usually all of the crosscuts between the entries will be sealed with stoppings to prevent the ventilation air currents from flowing from one entry to its adjacent entry, unless this is needed. These stoppings are effective barriers to a fire in one of the entries and will prevent the fire from spreading to the adjacent entry, at least for a reasonable time.
In contrast, the block system involves driving a larger number of parallel entries, often 8 or 10 and sometimes even more. With this multiplicity of entries, many will remain connected through open crosscuts and will share the ventilation. A fire in one of the connected entries will tend to stay in the originating entry as it will follow the ventilation, but with the expansion of the hot gases that have gone through the fire zone, the heat and smoke can move into adjacent open entries. Usually it is just a matter of time until the active fire will spread to adjacent entries, unless steps are taken to prevent this spread.
Current federal regulations require a mandoor in a line of stoppings at least every 500 feet. Regulations also require that entries for a conveyor belt, trolley haulage, and intake escapeway, with their ventilation, be separated by stoppings. Also, since many mines must leave pillars of coal to support the surface, all of the mining will involve the "driving" of entries and crosscuts. Where pillars can be removed and the surface lowered, many different arrangements of entries may be used to provide support of the strata over the entries needed for ventilation, travel, and production.
Other regulations require water hydrants at least every 300 feet in a belt entry. Usually this results in having the water line, that also supplies water to allay the dust made by the mining equipment on producing sections, located in the belt entry. Sufficient fire hose must be located on each section to be able to reach from the last hydrant at the end of the water line to the most distant point on the section. Also, fire hose must be located near the drive of the conveyor belts. Regulations also require fire protection systems near each belt drive to cover at least 50 feet of the belt drive area and a fire detection system for the length of each belt.
The record of fires clearly shows that the federal regulations have not eliminated the hazards of mine fires and losses of life and/or property resulting. The problems in coal mining are especially severe because the environment is almost 100% combustible.
The statistics of fires in underground coal mines are obviously inadequate as only fires that have existed for 30 minutes or more must be reported. However, discussions with safety managers have disclosed that most of them believe that a large percentage of the incipient fires that do occur are extinguished by the personnel present in less than 30 minutes. Estimates given in these discussions vary widely with many saying that 70 to 90 percent of the fires are handled quickly so no reporting was required.
Many reports of fires which did burn well over 30 minutes describe scenarios of the fire getting beyond the range of the fire hose streams. Prior to 1960 virtually all of these fires had to be sealed. Sealing is very expensive as the mine must remain sealed, often for four to six months, to allow the heat of the fire to cool and avoid rekindling when ventilation is restored in the fire area. In a 1960 fire the new tool of high expansion foam, conceived in Great Britain and developed and tested in the United States, controlled a fire that had spread over an area so that 4100 lineal feet of entries and crosscuts were burning. The foam equipment operated almost continuously for three days. This fire probably was the largest mine fire that was ever controlled by direct fire fighting.
Unfortunately only about 60 of the large high expansion foam generators similar to the one used on the above described fire have been purchased by the coal industry. They are not inexpensive as the current price for the generators, which must be tailor made for the mine, is in the order of thirty to forty-five thousand dollars. Many smaller and less refined generators are commercially available. Based on the fire reports available in the United States, these smaller and less expensive machines have never been successful in controlling a fire that spread to the coal. The result has been that some fires were not fought successfully using this advanced firefighting technique because the foam equipment was unavailable, inadequate, or mishandled.
It is also important to consider the manner in which nonreportable fires have been controlled and also why these techniques have failed in other instances. Because quick response to a fire is so important, virtually all fires that are extinguished are fought by persons who were at or in the vicinity of the fire when it started. If extinguishers are used quickly enough, they can extinguish the fire while it is still small. When the fire spreads to the coal, a fire hose stream can extinguish the fire if it has not spread beyond the range of the hose streams. The rate at which the fire will spread to the coal is greatly affected by the size of the starting fire, the height of the coal seam, the presence of a coal top or "roof", and the percent volatile of the coal.
Fires seldom get out of control in low seam mines, which usually have noncombustible roof. If the volatile content of the coal is low, the coal will not ignite readily and the rate a fire will spread is slow. These conditions which slow the spread of the fire provide more time to assemble the personnel and equipment needed to fight the fire. The advance of the firefighters is usually fast enough so that they can overtake the leading edge of the fire and extinguish it.
In contrast, the opposite will occur in coal mines having thick seams with high volatile coal, especially at the mine roof. The coal ignites readily, and the fire grows and moves downwind rapidly. This fire situation must be handled very quickly and competently with fire hoses before it gets beyond the range of a hose stream.
Since the maximum range of a fire hose adjusted to a solid stream in high coal is in the order of 40 to 50 feet, the fire will be large before it gets away from the fire hose streams. Because the fire is large and active, the downwind side of the fire is very hot with dense smoke. In the past attempts have been made to set a fire hose in a crosscut on the downwind side of the fire close to the entry to stop the fire, but the published fire reports indicate that these attempts have not been successful in difficult fire situations.
It is important to understand the conditions that make it difficult to fight fires in a coal mine. The amount of air that reaches the upwind side of the fire is the amount needed to provide safe conditions for the firefighters who are using the fire hoses. Usually, the ventilation air velocity must be in the order of 200 to 300 feet per minute (fpm) to prevent the smoke from the fire from moving back against this fresh air. Higher velocity will feed the fire and cause it to grow rapidly. Air velocity of 200 to 300 fpm is sufficient to feed a large fire.
In passing through the fire area, much of the oxygen is used by the fire and turned into gaseous products of combustion including smoke and heat. Usually the temperature on the downwind side of the fire is in excess of 1000.degree. F. (538.degree. C.). This temperature is enough to cause the gases to expand to almost three times (or more if the temperature is higher) the original air volume. Steam from the water reaching the fire may tend to keep the temperature down but it adds to the downwind volume. So the velocity of the gases leaving the active fire area can be three or more times the velocity of the fresh air on the upwind side of the fire.
In addition to the smoke and heat on the downwind side, the velocity can be 600 fpm or more. Velocities of this magnitude are not usually encountered by professional firemen wearing special heat protective gear. In view of these factors combined with the lack of professional firefighting equipment, the known methods for fighting an underground mine fire from the downwind side of the fire have not been successful.
In the text Mine Fires by Donald W. Mitchell, published 1990, on page 3 it states "one of the first critical steps when fighting fire in a mine is to SPRAY WATER (preferably as fog) INTO THE PATH OF (as close as possible to) THE ONCOMING FLAMES. The sooner this is done, the greater the chance for success." Unfortunately the Mitchell text does not tell how to put this water spray "into the path of the oncoming flames" but suggests that this be accomplished before the fire gets very large. The Mitchell text does not make reference to any successful uses of the disclosed technique and a search of the more current literature on this subject does not include any record that this firefighting technique has been successfully practiced.
Some mines have prepared lances with a nozzle attached to the end of a 20-foot long length of pipe. These have been used with some success in attacking fires on the upwind side when falls of the roof strata interfere with fire hose streams being directed at the fire which has spread beyond the fall. The lance can be laid on the top of the fall or in a crack of the fall and be pushed further into the fire area. Since the length of pipe that can be pushed over or through a fall is limited, a lance applied from the upwind side of a fire can only be effective on a fire that has not spread very far.
Consideration has been given to the use of lances on the downwind side of a fire. The plan of use for a lance is to make a small hole near the top of the stopping in the crosscut inby the fire and insert the lance and push it as far as possible to get close to the entry. In practice, there are two very difficult related problems with this scheme. First, virtually every mine has the line of stoppings in the crosscuts connecting with the entry containing the belt conveyor at some distance from the belt entry. So the usual distance from the stopping to the belt entry is about 50 to 70 feet. The reason for the stopping being closer to the other end of the crosscut is that it is easier and less disruptive of coal production if the material (usually concrete blocks) to build the stopping is delivered through the entry away from the belt entry. The stopping which must be built by hand is built close to where the material is delivered at the other end of the crosscut.
Second, since the lance must be cantilevered from the stopping, about the greatest distance that it is possible to do this with 11/2- or 2-inch pipe is one pipe length or 20 feet. The use of lances generally do not work in the most serious fire risk, a belt fire, which consistently causes more fires than any other. Similar to the scheme of stopping a fire as described above, there are no records of successfully using lances in stopping the spread of a fire on the downwind side of a fire. It is believed that this firefighting technique might be successful if the stopping were sufficiently close to the entry involved.
U.S. Pat. No. 3,684,021 discloses apparatus for detecting and suppressing a potentially dangerous flame from moving away from a mine face and down a mine tunnel. A flame extinguishing agent is not discharged upon the occurrence of ignition of small pockets of either methane or coal dust adjacent the mine face. As a rule most of the flames ignited adjacent the mine face terminate quickly or are extinguished by water from spray equipment mounted on the mining machine for suppressing dust and maintaining the cutter bits cool during the material dislodging operation.
As disclosed in U.S. Pat. No. 3,684,021 once the flame progresses from the mine face and advances in the entry where it begins to be fueled by ventilation air, the advancing flame is detected. A flame suppressing agent is then discharged ahead of the advancing flame. The agent completely fills the cross-sectional opening of the entry. When the flame reaches the area of the entry filled with the extinguishing agent, the flame is quenched and prevented from igniting an explosive mixture of airborne coal dust and/or methane gas.
U.S. Pat. No. 924,599 also discloses a method of extinguishing fires in a mine by sealing off the advancing flame. Upon the occurrence of a fire, the entrance to a heading is sealed by closing a door and a network of pipes are opened to the entry. An exhaust pump is connected to the network of pipes so that air is drawn out of the entry into the pipes. Consequently the atmospheric pressure in the sealed entry is reduced. The air normally supporting combustion is withdrawn and the fire is extinguished. The network of pipes can also be used to introduce steam to the entry to contribute to the fire suppression.
While it has been proposed to attach a nozzle or spray head to the outlet end of fire hose or pipe and advance the pipe outlet into a confined structure to flood the structure with water to suppress the flame, the known devices, as disclosed in U.S. Pat. No. 2,747,933, are not adaptable for use in fighting fires in an underground mine. It is also known to mount pipes or conduits on wheels to move the conduits to a desired position where water is sprayed from nozzles spaced along the length of the mobile pipe. These types of devices are most commonly utilized in lawn sprinkling systems as disclosed in U.S. Pat. Nos. 1,191,643; 1,282,142; 1,368,269; and 3,807,635.
U.S. Pat. No. 2,769,664 discloses a mobile sprinkler-type irrigation system. This system permits water to be conveyed through a network of pipes to a distance point where water is dispersed in a spray pattern from spaced-apart nozzles. The systems are not adaptable for use in an underground mine to suppress a fire.
While it is not possible to be certain that the methods or equipment disclosed in the above patents have ever been tried or used in coal mines, it can be stated safely that they are not in use now.
An obvious method that would virtually eliminate all risk of fires would be to install automatic sprinklers throughout the mine as the entries and crosscuts are being mined. However, the magnitude and cost of such an installation would be prohibitive.
As previously noted federal coal mine regulations require installed fire protection for only fifty feet of the mine opening containing the drive machinery of each belt conveyor. This regulation is aimed at the most serious fire risk areas in the mine. The regulation recognizes that built-in fire protection can only be justified for the most serious fire risks.
There is need to provide a method using simple equipment for creating a water barrier or curtain of water spray that continuously wets the roof and ribs of the mine entry on fire and to cool the hot gases of the fire so that the fire will not advance downwind. This must be accomplished without exposing any firefighters to the hot gases or smoke. By spraying water against the roof and with the pattern being wide enough to wet the ribs, these surfaces should be kept wet so that the flames and heat from the burning coal upwind will be unable to advance onto the wet coal. With ample water wetting the roof, the excess will drop back to the bottom of the entry resulting in further cooling of the hot gases and wetting of the bottom and any objects such as a conveyor belt in the entry.
Therefore, there is need for firefighting equipment capable of stopping a mine fire on the downwind side with a continuous application of water. With this method, the fire will not be able to continue to spread downwind. With the spread of the fire halted, there will be ample time for the firefighters to extinguish the fire using fire hoses in the fresh air on the upwind side of the fire.